Conventionally, in welded steel structures such as tower tanks, spherical tanks, baths, vessels (hereinafter, simply referred to as “vessels”), since aged deterioration due to exterior corrosion occurs, detection and repair by regular thickness measurement have been required.
As means for measuring the thickness of vessel steel plate, the measurement of the thickness of vessel steel plate has been performed by an ultrasonic probe. For example, in order to measure the thickness of flat bottom plate of a cylindrical tank, means for arranging ultrasonic probes and eddy current sensors are arranged in a staggered manner (alternately) on a traveling carriage, and a continuously measuring a steel plate thickness of the tank bottom plate flat surface by traveling the traveling carriage on a coating over the bottom plate surface of the cylindrical tank (see patent document 1) is proposed.
In addition, means for arranging ultrasonic probes in a staggered manner (see patent document 2), means for arranging ultrasonic probes along a width direction and performing thickness measurement of the tank bottom plate via a universal joint mechanism (see patent document 3), and means for supporting ultrasonic probes on a lifting mechanism via a gimbal joint (see patent document 4) are proposed.
Further, in order to measure a side plate curved surface of a floating roof tank, there is means for providing a guide traveling in a horizontal direction near the top of the tank side surface and bottom, connecting a measuring carriage on which ultrasonic probes and permanent magnets for absorption are mounted to a magnet wire rope, and lifting and lowering it by a cable take-up device (see patent document 5).
Further, various kinds of technologies such as means for arranging a traveling carriage on which ultrasonic probes are mounted to be movable vertically and horizontally (see patent document 6), and means for arranging ultrasonic probes to be movable in X-Y direction (see patent document 7) are proposed.
Patent Document 1
Japanese Patent Application Laid-Open (JP-A) No. 2001-50736
Patent Document 2
JP-A No. 2-194355
Patent Document 3
U.S. Pat. No. 5,440,929
Patent Document 4
JP-A No. 5-26654
Patent Document 5
JP-A No. 8-304062
Patent Document 6
JP-A No. 6-347250
Patent Document 7
JP-A No. 11-19890
In the case of a flat bottom plate of a large size tank simply intended for accommodation of contents or a vessel barrel at which no obstruction exists, effective thickness measurement is possible by the above described various conventional examples, however, in the case of a pressure vessel such as a reactor (reaction vessel), because the vessel is smaller compared to the large size tank simply intended for accommodation of contents, and further, there are many obstructions such as an agitator, agitating blades, a baffle as a filing pipe also serving for accelerating agitation, a gas suction pipe, and a thermometer within the vessel, and the vessel mirror part is formed by a spherical or conical curved surface, automatic entire thickness measurement using an ultrasonic flaw detector has been difficult, and visual detection from the outer surface side of the reactor has been generally performed.
Further, depending on the reactor, sometimes a jacket steel material for circulating hot water or water is provided around the outer circumference of a shell main body for the purpose of heat retaining or temperature adjustment of reaction temperature. While the part between the jacket steel material and the shell main body is exposed to a corrosive environment by the water environment, the outer surface of the shell main body at the part where the jacket steel material has been provided is covered by the jacket steel material. Accordingly, there are problems that the visual detection is difficult from the outer surface side of the shell main body, and a vast amount of cost is needed for once removing the jacket steel material and performing the visual detection, and restoring the jacket steel material again.
In this case, it is conceivable that, once the obstacle within the reactor is removed according to need, temporary scaffolding is provided within the reactor, and the thickness of vessel steel is measured manually using the ultrasonic flaw detector, however, if once the obstacle within the reactor is removed, the work on restoration is complicated and the work on providing and removing the temporary scaffolding is troublesome.
Further, in the case where the thickness of vessel steel is measured in a partial range manually using the ultrasonic flaw detector with the obstruction left within the reactor, because an inspector must perform thickness measurement in tight space in an unstable position, the working environment is bad, and because it takes a long time to measure the thickness of the entire surface of vessel steel without omission, the case is impractical for vessel operation. Accordingly, the thickness has been measured at representative parts of the vessel.
However, since reliability is poor in grasping a state of reduced thickness of the entire vessel only by measuring the thickness at representative parts of the vessel, sometimes the jacket steel material is once removed and the visual detection is performed from the outer surface side of the shell main body as described above.
On the other hand, in a multichannel thickness measuring device using plural ultrasonic probes, as shown in FIG. 23, a range to which intended reflection echoes return is assumed in advance and a boundary surface echo monitoring gate 41 and a bottom surface echo monitoring gate 42 are fixed, thickness values calculated in position where the threshold level of the bottom surface echo monitoring gate 42 cuts the reflection echo waveforms are used as measurement results.
However, in the multichannel thickness measuring device using ultrasonic probes, such a thickness measuring method can not deal flexibly with changes in damaged conditions of measurement surfaces of the independent channels and an object to be inspected, and problems such that accurate thickness values are overlooked and unnecessary noise is erroneously detected occur and those cause great errors in thickness measurement.
For example, in 1 ch (channel) to 4 ch in FIG. 23, thickness values can be accurately detected because the starting time point of the bottom surface echo monitoring gate 42 and the rising time points of bottom surface echo waveforms 43b are substantially matched, however, in other channels, since the thicknesses are calculated in the positions where the bottom surface echo monitoring gate 42 cuts the bottom surface echo waveforms 43b at the falling parts thereof, the thickness values thicker than the real values are detected. Therefore, it is an example in which thin thickness can not be measured and overlooked.
In order to deal with variations in measurement among many channels, as shown in FIG. 24, the conventional fixed gate system can deal with them by broadening the monitoring range of the bottom surface echo monitoring gate 42, however, in this case, there is a disadvantage that unnecessary noise becomes easier to be detected.
For example, in FIG. 24, since the thickness is calculated in the position where the bottom surface echo monitoring gate 42 that has dealt with the variation by broadening the monitoring range cuts a multiple echo waveform 43c in mistake for the bottom surface echo waveform 43b, thickness values thinner than the real values are detected.
Further, in the case where a vessel steel is formed by bonding different materials such as clad steel (e.g., SUS+SS material) and a steel material coated with glass lining (GL+SS material), if a separation (air layer) is produced at the respective boundaries, accurate entire thickness of the target material becomes difficult to be obtained and the separation causes great errors.
FIG. 28(a) shows a boundary surface echo waveform 43a in the case there is a separation at the boundary surface between the surface layer of a vessel steel plate formed by bonding surface layers of different materials and the vessel steel plate. If there is separation in the path of ultrasonic wave, since the multiple echo waveform 43c that is reflected and returned from the separation surface at plural times enters the gate range of the bottom surface echo monitoring gate 42, and the thickness is calculated in the positions where the bottom surface echo monitoring gate 42 cuts the multiple echo waveform 43c, the thickness values thinner than the real values are detected.
Further, in the case where inclusions and lamination exist in the steel material of the vessel steel, since, before the bottom surface echo waveform 43b appears, a flaw echo waveform 43d thereof emerges as shown in FIG. 28(c), the bottom surface echo monitoring gate 42 generally detects flaw echo waveform 43d instead of the bottom surface echo waveform 43b and greatly thinner thickness than the real steel plate thickness is detected as shown by 8 ch (channel) in FIG. 31. Accordingly, there is a problem that, with respect to the steel material with no corrosion, a thin thickness value 44 as if it is corroded is calculated and displayed, and the corrosive reduction of thickness of vessel steel and existence of inclusions etc. can not be discriminated.
Further, in a vessel provided with a jacket steel material on the outer circumference, even when the steel plate thickness is measured from inside using ultrasonic probes, the positions can not be located easily from outside, and, it is necessary to roughly locate the position required for repairing from the outer side from thickness information that has been measured at the inner side, and repair the broad range around the located position. Thereby, there has been a problem that the vessel strength becomes deteriorated by greatly removing the jacket steel material, and time and cost are required for repairing in the broad range.